Like several people, I can't find a specific spot where the shadow character would structurally fit. This is the character who's a representation of what the central character will face unless he succeeds. If there were a label for this, Consequence Character would be nice. That implies the OS is where to look for fitting this in. If several minds have put this type into their stories, I'd like to think there's some inspiration for it. It could be that the Main Character has Impact Character guiding MC one way and this other character showing the other way is not the proper course. But that would be Subjective Story.